F18s My First Exposure

[Bill Roberts]

At the 2013 Steeplechase I saw F18s up close for the first time. Now I know what the "box rule" means. It means that the hull cross section should be a box or very close to a box, maybe a slight curve from chine to chine. For years the hull botttoms on any and all performance catamarans was a rounded or semicircular or elliptical hull shape. These F18s have made a major departure from that to these box hull shapes with rounded corners. Well, what's going on here? What's the logic?
Let's do a little study and compare a box type hull cross section to a rounded hull cross section of the same displacement per foot of hull length. Let's go to a hull section of max size at or about the main beam. These F18s that I saw were very wide hulls. In our simple example lets assume that with one hull carrying the total boat weight, hull flying, our box rule hull shape underwater is 2ft wide by 1ft deep at this max section. The underwater area is 2ft sq and the wet perimeter is 4ft. The underwater hull cross sectional area is proportional to displacement. Simply multiply this area by 1ft of hull length and we have 2 cu ft of hull volume which will support 2 X 64 lbs per cu ft or 128 pounds. The wetted area of this hull is simply 4ft X 1ft = 4 sq ft of wetted area or drag making area.
Now let's go to the semicircular hull shape. Again the hull is 2ft wide with a 1ft radius. The underwater area is 1/2 X Pi X 1**2
and this equals 1.57 sq ft. Well the box hull had 2 sq ft underwater so we have to add a little rectangular area on top of the semicircular area to get it up to 2 sq ft underwater. So we have to add 0.43 sq ft of area on top of a 2 ft wide base. The height of this added rectangular area is 0.215 ft or 2.58 ins. Now both hull shapes have the same underwater area and therefore the same displacement per 1 ft of hull length. Now the perimeter of the semicircular hull shape is 1/2 X Pi X D = 3.14ft plus 2 X 0.215 ft = 3.57ft. The wetted area of this hull shape per foot of hull length is 3.57 sq ft. Well how about this, the rounded hull sahpe of the same displacement has 10.75% less wetted area or drag making area than the boxey shape. Now we understand why well rounded hull bottom shapes on catamarans have been popular on performance boats for many years.
I talked to some F18 owners and asked about the boxey hull bottom shape and I was told that the design is a semi planning hull shape. I have first hand experience with planning hull shapes on hydroplanes and one of the most important points to the hull shape is "hard chines". Hard chines capture the vertical component of the energy in the spray and make it lift the hull. Round chines let that energy escape. The logic being that it costs more drag to capture that energy than you get back. Therefore round hull bottoms and no hard chines go together. But flatish hull bottoms and hard chines might go together OK, especially if the boat is fast enough to plane or semi plane.
Another thing on hull shapes that was pointed out to me on one of the F18s was a slight kick down in the keel line/flat bottom just in front of the transom. This is a power boat thing. Power boats do this to stop them from porpoiseing while planning. If a power boat hull naturally has a tendenct to porporise, this can be stopped by making a slight hollow/concave shape in the hull just in front of the transom. It can also be stopped by putting downward trim on trim tabs at the transom. Either one of these correction methods comes with a slight increase in drag and reduces the top speed of the boat.
So, this is my first technical experience with F18s and I'm not too well impressed.

[Bruiser]

Interesting stuff. Bill do have any info comparing the sail plans of the F 18 (mast height, main sail spin) verses SC 17 or ARC 22. I am curious even how they compare to older Hobie 20's 18's etc. I am guessing they are getting their increased performance out of sail to horsepower ratios rather than hull designs? (Granted they got their butts kicked by the 22) Just wondering.....

[Bill Roberts]

Hi Rich,
I don't have any specs on F boats but surely there is a web site for the F box rule boats/classes. The more I think about beach cat racing growth, maybe this is the way it has to be in the future. I don't see one manufacturer coming out with a magic boat that everyone wants to buy. Therefore the size and number of manufacturers is shrinking. The box rule scheme allows interested manufacturers to build so called one design boats with slight variations and everybody gets a little of the market. No longer is their a builder strong enough to build boats and sell them at little or no profit for a few years while the class grows and then down the road raise the price and make big profits, a la Laser.
One could make an 18 ft boat out of the SC17 or SC19 molds. He could make a 16ft boat out of the SC17 or SC15 molds. All boats would have daggerboards. The sail plans would have to be to the F class rules. I guess Tom does not want to get in that cat fight and his phone isn't ringing off the hook with hungry customers. SC and ARC and RC beach catamarans are the highest performance boats in their sizes ever built. If F class sailors want a faster F boat, they should go to Tom. I don't understand why they don't ask him to build F class boats.
I guess they just like the sound of "boat built in China". It must be better.